Supreme Court Chief Justice John Roberts decided to save President Obama’s signature legislation by ruling that the individual mandate requiring Americans to have health coverage or pay a fine is constitutional under the federal government’s taxing power.

That ruling took most people by surprise, since the Justice Department appears to have thought the “taxing power” argument was the weakest of its three weak justifications. Maybe Roberts has an affinity for weak arguments, because he certainly made a number of them himself in justifying the mandate as a tax.

When the chief justice and his liberal colleagues consider the mandate a tax, they seem to mean only the penalty a person must pay for not having health insurance. However, the mandate has two parts: the requirement to have coverage and the penalty — er, tax — for not getting it. Why aren’t both considered a tax?

ABC’s George Stephanopoulos did. When he directly challenged the president on the mandate-as-tax question, he said, “Under this mandate, the government is forcing people to spend money, fining you if you don't. How is that not a tax?”

And if both the premium and the penalty are considered a tax, the mandate becomes the largest tax increase in U.S. history. And that doesn't include all of the other taxes imposed by the legislation.

The median U.S. family income is about $50,000. Family health coverage can easily run $20,000 a year — and rising quickly. In that scenario, the coverage mandate is essentially a 40 percent tax on that family, which is now required by law to ensure that every family member has qualifying coverage.

And because the cost of the coverage will be similar even though incomes vary significantly, the lower the income the higher the effective tax rate — in essence, the most regressive tax in U.S. history, too.

Now, President Obama and his enablers may deny the health insurance premium is a tax. But the bill’s defenders — including Budget Director Peter Orszag, Health and Human Services Secretary Kathleen Sebelius, and the president himself, not to mention Democrats in Congress — also denied the mandate was a tax.

But when it became useful for the law’s supporters to consider the mandate a tax, they made the mental switcheroo easily. That’s because consistency and truth were not the goal; getting five Supreme Court votes was the only thing that mattered.

If Roberts had told the liberals on the Court that the price for him signing on to the mandate was to consider both the premiums and the penalty a tax, would any of them have objected? How about the Flip-Flopper in Chief?